THALLOPHYTES 261 



The Algce are uni- or multi-cellular, chlorophyll-bearing thal- 

 lus plants. They are mostly aquatic plants and are found in 

 nearly every stream, pond, lake and sea throughout the world 

 and in waters ranging in temperature from near the boiling 

 point to the waters of the polar regions. A comparatively few 

 species live on wet soil and rocks and a very few live on the 

 bark of trees and in other places where the conditions are very 

 dry for a considerable part of the time. Since they contain 

 chlorophyll they must receive sunlight. They are of little direct 

 economic importance. Some few are used for food, especially 

 those growing along the coast of Japan ; others have been used 

 as a source of potassium, especially the very large marine forms 

 of the American Pacific Coast ; they are also an important source 

 of iodine and a few other chemicals. However, when we take 

 into consideration that they form one of the important sources 

 of food, either directly or indirectly, for fishes and other aquatic 

 animal life, we realize that it is extremely difficult to appreciate 

 their true value. While fishes may not use the algte directly to 

 any great extent, they feed on other animals which have fed on 

 the algse. The young of most species of fish feed entirely on 

 micro-organisms, both plant and animal. But the algse are inter- 

 esting to us from another and entirely different viewpoint. It 

 is very evident that algse or some very similar plants must have 

 been the first forms to appear on the earth and that all other 

 forms of life must have been derived from them. Therefore, 

 a study of the algse is very important in order to gain an under- 

 standing of organic evolution. 



The Cyanophycea? or Sdiizophycece (Fig. 118) are the 

 " blue-green " or " fission algse " and are characterized by the 

 blue-green color due to a combination of phycocyanin and chloro- 

 phyll and to the fact that they reproduce by simple cell division. 

 They are the simplest known species of the algse, are microscopic 

 in size, although they frequently form very conspicuous masses, 

 and are very widely distributed throughout the earth. They are 

 mostly aquatic and live in water ranging in temperature from 



